Lithography is the process of transferring an image from a photomask onto a silicon wafer. Since 1971, advances in lithography have allowed integrated circuit (IC) manufacturers to reduce minimum feature sizes from 10 microns to 0.13 microns. This steady miniaturization has enabled improvements in IC performance and growth in the semiconductor industry.
I.A The Lithography Process
The primary lithography tool is a wafer stepper, which generally consists of a light source, a photomask holder, an optical system for projecting and demagnifying the image of the mask onto a photoresist-coated wafer, and a stage to move the wafer. Lithography also requires a photomask, a quartz substrate with chrome lines on one surface. The chrome lines form a perfect master of the pattern to be inscribed on one layer of a chip.
The stepper shines light from a laser onto the photomask. A 4× reduction lens focuses the light to form an image on a silicon wafer. This image consists of a pattern of light and dark—light where the photons passed through the bare quartz; dark where the chrome lines cast shadows. The light exposes photoresist, a photosensitive chemical, on the wafer.
The industry uses two types of photoresist, referred to as “positive” and “negative.” Positive photoresist is insoluble when first applied, and the light “softens” it, or renders it more soluble. Negative photoresist has the opposite characteristics: it is soluble, when first applied, and the light “hardens”, it or renders it more soluble.
In both cases, the next step is to “develop” the photoresist; i.e., to apply a solvent known as a developer to remove the more soluble photoresist. The less soluble photoresist remains on the wafer to protect the underlying material from a subsequent processing step. This processing step can be an etch to remove material that is unprotected by photoresist. In this way, lithography transfers the pattern from the mask onto the wafer. By repeating this technique several times using different photomasks, it is possible to build multi-layered semiconductor structures (e.g., transistors) and associated interconnecting electrical circuits.
I.B The Lithographer's Equation
Within the limitations of this prior art lithography paradigm, the “lithographer's equation” determines the resolution, or the smallest feature that can be inscribed on the wafer: Resolution=k1×λ/NA, where λ (lambda) is the wavelength of the stepper light, NA is the numerical aperture of the stepper lens, and k1 is a “process factor” which several contributors can influence. The lithographers' equation says that there are three ways to write smaller features on a wafer:                Decrease the exposure wavelength. Since 1986, new generations of steppers have reduced the exposure wavelength from 468 nm (the “g-line” of a mercury vapor lamp) to 365 (the “i-line of a mercury vapor lamp), to 248-nm (light from a krypton fluoride excimer laser, and most recently to 193 nm (light from an argon fluoride excimer laser).        Reduce the k1 factor. This catch-all category embraces everything that lithographers can do to improve resolution without decreasing the exposure wavelength or increasing the numerical aperture of the stepper lens. For example, lithographers can reduce k1 by adjusting the orientation or the shape of the illumination beam. The most important ways of lowering k1 have required changes in the photomask. Sub-wavelength resolution requires masks with optical proximity correction and phase-shift techniques.        Increase the numerical aperture of the stepper lens. The NA of the stepper lens is the sine of the angle between the normal and the outside of the cone of light striking the photomask. It is possible to print smaller features by increasing the NA of the lens. A higher NA requires a larger, more expensive lens.        
Table 1 shows how the semiconductor industry and its suppliers have changed the exposure wavelength (λ), k1, and the NA of the stepper lens over the past 15 years.
TABLE 1Evolution of k1, λ, and NALinewidthλYear(μm)(nm)K1NA19861.04361.00.3819890.73650.86.4519920.53650.74.5419950.353650.57.6019970.253650.50.6019990.182480.43.7020010.132480.39.75I.C The State of the Art
Today semiconductor manufacturers are creating circuits with 130-nanometer features by exposing wafers in steppers with 248-nm light from krypton fluoride excimer lasers. They achieve the low k1 factor of 0.39 with the aid of special masks which contain phase shifters and optical proximity correction features. The key point is that in commercial-scale production, the industry has found no way to expose features which are smaller than approximately half the exposure wavelength.
I.D The Semiconductor Industry's Lithography Roadmap
Table 2 shows the semiconductor industry's lithography roadmap for the future, according to International SEMATECH.
TABLE 2Semiconductor Industry's Lithography RoadmapMinimum FeatureSize (nm)YearExposure Technique1302001Optical, 248 nm krypton fluoride laser902004Optical, 193-nm argon fluoride laser652007Optical, 157-nm fluorine laser452010EUV Lithography and/or EPL
Table 2 indicates that the industry will introduce 193-nm optical lithography for the 90-nm generation of integrated circuits in 2004 and 157-nm optical lithography for the 65-nm generation of integrated circuits in 2007. Both of these techniques expose photoresist with deep ultraviolet light from lasers.
The semiconductor industry's roadmap assumes that optical lithography cannot produce circuits with minimum feature sizes much below 65 nanometers. Semiconductor manufacturers expect to adopt either extreme ultra-violet (EUV) lithography or electron projection lithography (EPL) for the 45-nm generation of integrated circuits.
EUV lithography exposes the wafer with 13.4-nm radiation (soft X-rays). Since virtually no materials are transparent to radiation at that wavelength, the technology requires reflective masks and lenses. EPL exposes the wafer with electrons rather than with photons. Industry consortia are investing heavily to develop EUV and EPL lithography. Both technologies involve substantial technical risk, schedule risk, and economic risk. Furthermore, they may significantly increase the costs of lithography. For these reasons, the industry will have great interest in new methods of optical lithography which would enable steppers with optical wavelengths (248 nm, 193 nm, and 157 nm) to produce smaller features than presently expected.
To provide a foundation for the discussion of our novel methods, we now turn to the properties of conventional photoresists.
I.E Properties of Conventional “Integrating” Photoresist
A conventional photoresist changes its solubility when exposed to light. For example, FIG. 1 shows how the solubility of positive photoresist changes as it receives increasing amounts of light energy (photons per unit area). As the energy increases from zero, the solubility remains low at first. Then as the applied energy approaches some threshold (E0), the solubility increases quickly to some high level and then levels off. The transition from insoluble to soluble occurs over a rather narrow range of energy ΔE.
The time over which the photons strike the photoresist is not a factor. The curve shown in FIG. 1 is valid regardless of whether the photoresist receives low-intensity light for a relatively long time or high-intensity light for a relatively short time. Furthermore, the application of light need not be continuous. If the photoresist receives only a sub-threshold amount of light energy, say 50% of the threshold value, or E0/2,it “remembers” that energy. Another dose of energy equal to E0/2 applied at some later time will catalyze the photoresist (to “catalyze” photoresist means to change its solubility). We refer to resists of this type as “integrating” resists because they integrate the energy applied over time.
I.F Proof of Principle of 40-nm Features with 248-nm Lithography
It is possible to create 40 nm resist features with a 248-nm stepper by a chromeless phase shift approach.1 FIG. 2 represents the distribution of light which that approach projected onto the photoresist. The phase shift technique created a positive amplitude on one side and a negative amplitude on the other. The photoresist is sensitive only to the intensity of the light, which equals the square of the amplitude and is therefore a positive number. The intensity distribution shown contains a narrow sub-threshold region in the center. This region prints on the photoresist as a narrow dark line. This photoresist line has a width of 40 nm.
1 See M. Fritze, J. Burns, et al., “Sub-100 nm silicon in insulator complementary metal-oxide semiconductor transistors by deep ultraviolet optical lithography,” J. Vac. Sci. Technol, B 18(06), November/December 2000, pp. 2886-2891. 
From a practical standpoint, the main limitation with this technique is that it can create only semi-isolated gate features, not fully dense integrated circuits. Ideally, it would be useful to print densely packed lines and spaces in which the spaces had the same dimensions as the width of the lines. It would be impossible to print a second dark line close to the one shown in FIG. 2. The second dark line would have to be far enough away from the original dark line to ensure that the tail of its intensity curve would lie totally outside the original dark line. Otherwise, the light from the second exposure would add to the sub-threshold exposure within the original dark line and catalyze the resist in that area.
I.G Non-Integrating Photoresists
Researchers have demonstrated another category of photoresist which we refer to as “non-integrating” photoresists. If these photoresists receive a sub-threshold dose of light energy, they relax back into their original state after a characteristic relaxation time. For example, multiple doses of light below the threshold value will not catalyze this type of photoresist, provided only that these doses are separated in time by at least the characteristic relaxation time.
One class of non-integrating resist is resists which respond to temperature. These thermoresists (also called thermochroic resists) change solubility irreversibly once they have been raised above a certain temperature.2 If the resist is raised to an elevated temperature below the critical temperature, it is unchanged. This means that if light is used to heat the resist, only those areas of resist heated above the critical temperature will be exposed, and once the light is turned off, the resist will cool, so that the areas heated to below the critical temperature will effectively ‘forget’ that they were ever heated.
2 D. Gelbart, “248 nm thermoresist: 0.1 micron imaging without proximity effects,” in Emerging lithographic technologies III, Y. Vladimirsky, ed., Proc. SPIE v.3676, 1999. 
We will disclose a lithography strategy to exploit the properties of thermochroic photoresists and other non-integrative photoresists.